1. Field of the Invention
The present invention relates to a low-noise amplifier, and more particularly, to a variable-gain low-noise amplifier.
2. Description of the Related Art
For implementation of good communication in a long distance, output power of a transmitter may be designed to be increased, or a sensitivity of a receiver may be designed to be improved. However, the approach for increasing the output power of the transmitter is not preferable due to limitation of power level of an output stage of the transmitter, influence to associated apparatuses, and cost thereof. Therefore, the approach for improving the sensitivity of the receiver is preferably used. The sensitivity characteristics of the receiver can be expressed by a noise figure (NF) which represents a degree of extracting reception signal from noises. The smaller the NF is, the better the sensitivity of the receiver is.
Recently, portable communication technologies in frequency bandwidths of 400 MHz to 2.5 GHz have been rapidly developed. Therefore, technologies for radio frequency (RF) devices and circuits have also been important. In such an RF device or system, a low-noise amplifier (LNA) is a circuit for amplifying a very small signal received through an antenna of a receiver without noise. In general, since the low-noise amplifier is disposed at the first state of the receiver, the NF of the low-noise amplifier determines a whole performance of the receiver. Therefore, in the low-noise amplifier, noise and signal deformation need to be suppressed.
FIG. 1 is a circuit diagram illustrating a conventional low-noise amplifier.
Referring to FIG. 1, the low-noise amplifier 100 includes three inductors Ld, Lg, and Ls and three MOS transistors M1, M2, and M3.
The inductor Ls is an input-impedance matching inductor. When a frequency of an input signal is f0, an imaginary part of the input impedance is 0 (zero). At this time, the input impedance is matched to a desired value of 50Ω by adjusting the inductance of the input-impedance matching inductor Ls, so that maximum signal transmission can be obtained. The inductor Lg is a frequency-band inductor. A frequency band of operation of the low-noise amplifier is determined by adjusting the inductance of the frequency-band inductor Lg. The inductor Ld is a resonance inductor. The resonance inductor Ld together with a parasitic capacitance between a drain of the second MOS transistor M2 and a substrate and a gate capacitance of the third MOS transistor M3 constitutes a resonance circuit that resonates at a desired frequency.
The second MOS transistor M2 has a common gate amplifier structure in which reverse isolation between input and output can be increased. That is, an influx of a signal reflected from an apparatus which receives the output of the low-noise amplifier can be suppressed as large as possible. The feedback from the output to the input can be minimized, so that stability of the circuit can be improved. The third MOS transistor M3 has a common source amplifier structure in which gain can be increased.
The first MOS transistor M1, that is, an input transistor has a common source structure in which an inductor is inserted between the gate and the source. The input impedance Zi can be expressed by Equation 1.
                              Z          i                =                                            jw              ⁡                              (                                                      L                    s                                    +                                      L                    g                                                  )                                      +                          1                              jwC                gs                                      +                                          (                                                      g                    m                                                        C                    gs                                                  )                            ⁢                              L                s                                              ≈                                    w              T                        ⁢                          L              s                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Here, if Zi=50Ω by adjusting the value of Ls, the input impedance matching can be obtained. In this method, there is no need for connection to an external circuit, so that the method has an advantage in terms of noise figure (NF). The gain of the amplifier, that is, trans-conductance Gm is independent of bias, but it is correlated to only the input-impedance matching inductor Ls. [impedance→input impedance] The trans-conductance Gm and small-signal gain Av of the circuit can be expressed by Equations 2 and 3, respectively.
                              G          m                =                                            g              m                                                      w                0                            ⁢                                                C                  gs                                ⁡                                  (                                                                                    w                        T                                            ⁢                                              L                        s                                                              +                                          R                      s                                                        )                                                              =                      1                          2              ⁢                              w                0                            ⁢                              L                s                                                                        [                  Equation          ⁢                                          ⁢          2                ]                                          A          v                =                              -                          G              m                                ⁢                      R            L                                              [                  Equation          ⁢                                          ⁢          3                ]            
Referring to Equation 3, it can be seen that the small-signal gain Av of the low-noise amplifier that is subject to impedance and frequency matching is independent of bias, but it is inversely proportional to a signal frequency w0 and source inductance Ls.
In design of such a low-noise amplifier shown in FIG. 1, it is necessary to minimize the noise figure (NF), to ensure IP3 (third order intercept point), to lower input impedance, and to maintain suitable gain. Here, a small value of the NF denotes that a low-noise amplifier generates a small amount of noise. If the NF has a small value, a low level of signal can be amplified irrespective of influence of noise, so that a small-sized antenna can be implemented.
However, although the aforementioned electric characteristics are satisfied, the gain of the low-noise amplifier needs to be varied so as to widen applications range of the low-noise amplifier.